I'm so sorry you're having to go through this. Just talk with them again and if it really seems to be finanical reasons - and you have the money - just explain to them that this is your special day and it just wouldn't be the same without them and that you are going to pay their way. If they feel badly about taking your money, then tell them they can pay you back when things are better for them.
My FI and I have 2 grown children each, plus their spouse, and paid for their flights and hotel, however one of my daughters can't go because she's too far along in her pregnancy , but wanted to go, and his daughter just doesn't want to go because she says "it would be weird to see her father get married". (He's a widower.) However, his son is standing up as his BM and my other daughter is my MOH. Just can't please everyone.
I sincerely hope it works out for you!!